file:Kieselsaeure380m2prog.jpg Silica fume also known as microsilica is a fine-grain, thin, and very high surface area silica It is sometimes confused with fumed silica (also known as pyrogenic silica) and colloidal silica. These materials have different derivations, technical characteristics, and applications.

Properties

Silica fume consists of fine Glass particles with a surface area on the order of 215,280 ft²/lb (20,000 m²/kg) when measured by nitrogen adsorption techniques with particles approximately one hundredth the size of the average cement particle.lt;/ref>

Production

Silica fume is a byproduct in the reduction of high-purity quartz with coke (fuel) in electric arc furnace in the production of silicon and ferrosilicon alloys.

Applications

Concrete

Because of its extreme fineness and high silica content, silica fume is a very effective pozzolan c material.ACI Committee 226. 1987b. "Silica fume in concrete: Preliminary report", ACI Materials JournalMarch-April: 158–66.Luther, M. D. 1990. "High-performance silica fume (microsilica)—Modified cementitious repair materials". 69th annual meeting of the Transportation Research Board, paper no. 890448 (January) Standard specifications for silica fume used in cementitious mixtures are ASTM C1240ASTM C1240. Standard Specification for Silica Fume Used in Cementitious Mixtures, http://astm.org, EN 13263EN 13263 Silica fume for concrete. http://www.cen.eu. Silica fume is added to Portland cement concrete to improve its properties, in particular its compressive strength bond strength and abrasion (mechanical) resistance. These improvements stem from both the mechanical improvements resulting from addition of a very fine powder to the cement paste mix as well as from the pozzolanic reaction between the silica fume and free calcium hydroxide in the paste.Detwiler, R.J. and Mehta, P.K., [http://concreteinternational.com/pages/featured_article.asp?ID=2281 Chemical and Physical Effects of Silica Fume on the Mechanical Behavior of Concrete] Materials JournalNov. 1989 Addition of silica fume also reduces the permeability (fluid) of concrete to chloride ion , which protects the rebar of concrete from corrosion especially in chloride-rich environments such as coastal regions and those of humid continental roadways and runways (because of the use of deicing salts) and seawater bridges.lt;/ref> Prior to the mid-1970s, nearly all silica fume was discharged into the atmosphere. After environmental concerns necessitated the collection and landfill ng of silica fume, it became economically viable to use silica fume in various applications, in particular high-performance concreteACI 234R-06. Guide to Silica Fume in Concrete http://www.concrete.org American Concrete Institute]. effect of silica fume on different properties of fresh and harden concrete:- a) workablity:- with the addition of silica fume, the slump loss with time is directly propotional to increase in the silicafume content due to the introduction of large surface area in the concrete mix by its adition. although the slump decreases the mix remains highly cohesive. b) segregation and bleeding:- silicafume reduces bleeding significantly because the free water is used up in wetting of the large surface area of the silicafume and hence the free water left in the mix for bleeding also decreases. silicafume also blocks the pores in the fresh concrete so the inside water is not allowed to come to the surface of the concrete.

See also

* Engineered Cementitious Composite * Fly ash * Kaolinite * Pozzolan * Rice husk ash * Metakaolin

Notes

References

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External links

* http://silicafume.org/ Silica Fume Association] Category:Silicon compounds Category:Ceramic materials Category:Glass types es:Humo de sílice it:Microsilice he:מיקרו-סיליקה nl:Microsilica ru:Микрокремнезём